1. Technical Field
The present invention relates to a communication method and a communication system performing communication according to a protocol.
2. Related Art
When recording or transmitting digital data, it is necessary to perform signal conversion (modulation) on the digital data so as to have an appropriate data structure, which can be realized by, for example, providing binary data composed of “0” and “1” corresponding to each of the level of the amplitude, the level of the frequency, the phase angle, the flux direction, and so on. In this case, Non Return to Zero (NRZ) continuously reflecting them in the signal state, and NRZ Inversion (NRZI) keeps the state of “0” after generating a pulse corresponding to “1.” It should be noted that since these signals do not include a timing clock for determining “0” or “1,” it is required to separately prepare a clock signal for synchronization. As a method for such synchronization, there have been known two methods described below.
One is an NRZI method of adding a clock pulse to every bit. Since in this case two pulses are generated for every bit, the recording performance and the transmission performance are degraded by half.
The other is a method of using a data pulse generated in accordance with “1” directly as a clock pulse, and then outputting a timing pulse between consecutive bits on and after the second bit when “0” comes consecutively. In this case, in order for detecting the data and the clock, it is required to perform sampling at a double frequency when retrieving the data.
However, since it is not achievable for the two methods described above to raise the transmission density, as a method of improving the circumstance there is known a standard such as Run Length Limited (RLL) 2.7, RLL 1.7, Eight to Fourteen Modulation (EFM), 8-16 modulation as described in JP-A-8-154053. These standards each relate to a method of detecting a changing point (an edge) between “1” and “0” thus modulated as described above on the retrieving side, and generating the clock using a Phase Locked Loop (PLL) or the like, and thus the transmission density can be raised.
However, according to such a method, since it is required to perform the modulation on the data afterward and limit consecutive appearance of “0” or “1,” it is necessary to provide redundancy of 50% through 100% to the data expressed in a protocol 80A of a normal state as shown in FIG. 4 in order for providing the changing point of the bit value. For example, as shown in a communication protocol 80C added with redundancy, since the length of the data is elongated from 40 bits to 56 bits, an amount of data is increased, and improvement of the transmission density is inhibited.